The present invention relates to a method for reading recorded information from an optical disk, particularly a write-once optical disk which is subject to corruption due to ageing. It relates also to apparatus for performing the method.
Optical write-once discs, such as CD-R and DVD-R disks, are widely used for data storage. Such a disk presently has a cross-sectional structure as shown in FIG. 1. It is produced from a disk-shaped plastic substrate 3 which was formed with a xe2x80x9cpre-groovexe2x80x9d 5 extending in a spiral across its recording face (the upper one in FIG. 1). Subsequently, a layer 7 of dye is formed overlying the recording face, a reflective layer 8 (such as aluminium) is formed on the dye lyaer 7, and a protective layer 9 is formed on the reflective layer 8. The cross-section of the disk at this stage is as illustrated on the left of FIG. 1.
Data is written to the disk using a first laser beam (xe2x80x9cwrite laserxe2x80x9d) which illuminates the disk from below as viewed in FIG. 1, at selected locations. At portions of the pre-groove, such as that illustrated generally as 10, which have been illuminated by the write laser, the write laser alters the dye layer as shown in area 12, changing its reflective index. A subsidiary effect of the writing process is that the pre-groove is deformed, indicated schematically in FIG. 1 as configuration 11.
Portions of the pre-groove which were not exposed to the write laser (such as the area shown on the left of the figure) are referred to as xe2x80x9clandsxe2x80x9d. Portions which have been exposed to the write laser, such as portions 10, are called xe2x80x9cpitsxe2x80x9d. Due to the change in the reflective index of the dye as between a pit and a land, a second laser beam (xe2x80x9cread laserxe2x80x9d) incident on the disk (again from below as shown in FIG. 1) is reflected with a difference in optical path distance between a xe2x80x9cpitxe2x80x9d and a xe2x80x9clandxe2x80x9d. This difference makes it possible to detect the recorded information by an optical pickup.
A typical conventional write-read process is illustrated in FIG. 2(a). In step 21 digital data is input to a signal processing unit. In step 22 it is converted to a known standard (xe2x80x9cEFMxe2x80x9d or xe2x80x9ceight-fourteen modulationxe2x80x9d). In step 23, recording software uses the result to generate a pulse stream for switching the write laser on and off as the write laser scans along the pre-groove 5. Thus data is written to the write-once optical disk.
In a conventional read process, illustrated in FIG. 2(b), a read laser (which is of lower power than the write laser) scans the disk along the pre-groove, measuring the degree of reflection of the laser from the surface (step 26). The data is converted out of the EFM standard (step 27). Thus the digital data written on the disk is extracted (step 28).
Due to the inherent instability of the organic dye layer to light, the dye will decay over a period of years. When the dye decays, the recorded pattern within the dye can no longer be recovered by the read process of FIG. 2(b), leading to a potential loss of data. To address this problem, some researchers are trying to develop new organic dye layers which have a longer life.
In general terms, the present invention proposes that a write-once optical disk having data written on it, and subject to corruption so that the data cannot be read by a conventional read process, is read by removing the dye layer (and other layers thereon) to expose the surface of the substrate, measuring deformations to that layer caused by the write process, and extracting the data from the measured deformations.
Specifically the invention proposes a method for retrieving recorded information from an optical write-once disk comprising a substrate and a dye layer overlying a recording surface of the substrate, digital data encoded by a recording standard having been recorded on the disk by an optical write process, the method comprising the steps of:
removing the dye layer from the recording surface of the substrate to expose the recording surface;
measuring the topography of deformations of the recording surface to extract numerical parameters of sections of the disk;
using the extracted numerical parameters and the recording standard to classify the sections of the recording surface; and
extracting the digital data from the classification of the sections.
The disk is generally of the known type described above in which the data is recorded sequentially along a spiral pre-groove extending around the axis of the disk and from a radially inward position to a radially outward position. The sections are then sections of the pre-groove, and the measurement of the topography is a measurement carried out along the entire length of the pre-groove. In particular, the numerical parameters characterise deformations of the pre-groove caused by the write process.
Furthermore, the digital data is preferably written on the disk as a series of pits and lands of predetermined types associated with the recording standard (e.g. EMF). The classification includes classifying sections of the optical disk corresponding to one of these types of pit or land.
The extracted numerical parameters preferably include at least the lengths of lands and of the deformations corresponding to pits. They may further include the widths of deformations corresponding to pits, and/or any other numerical parameters of the topography associated with pits.
Suppose that the classification process employs a certain number (at least one, but not necessarily all) of the numerical parameters extracted from the topography. The classification can then be considered as being based on a plurality of regions defined in that space and corresponding respectively to the predetermined patterns. Each section of the pre-groove is classified by determining which of the regions the parameters extracted for that section of the pre-groove falls into. The process of selecting the classification process thus corresponds to selecting which of the extracted parameters to employ in constructing the parameter space, and where the borders of the regions should be located in that space. The second of these processes may be performed by observing how extracted numerical parameters are clustered in the space. The regions should be selected such that each corresponds to a cluster. Preferably, this process is performed automatically.
For example, the classification process may be performed using a neural network as a classification tool. The parameters of the neural network may be tuned according to known algorithms to set the borders of the regions in the parameter space.
It is possible to test the classification process to determine the accuracy of the classification process. For example, the accuracy of the classification process is related to the degree to which the regions are well separated. If it is found that a high proportion of the deformation patterns are close to two regions, then the accuracy of the classification process will be low, and the classification process may be modified. For example, the definition of the regions in the parameter space may be changed. Alternatively, a different parameter space may be employed, i.e. with different one(s) of the extracted parameters and/or a greater number of the extracted parameters.
The measurement of the topography of the recording surface may be performed by atomic force microscopy.
As mentioned above, a known disk typically comprises one or more further layers, i.e. reflective layer(s) and protective layer(s), covering the dye layer and thus overlying the recording surface of the substrate. In the case of such a disk, the removal of the dye layer further includes removing the further layers.
Furthermore, the invention proposes an apparatus for carrying out the method according to the invention. Specifically, the invention proposes an apparatus for retrieving recorded information from an optical write-once disk comprising a substrate and a dye layer overlying a recording surface of the substrate, digital data encoded by a recording standard having been recorded on the disk by an optical write process, the apparatus comprising:
means for removing the dye layer from the recording surface of the substrate to expose the recording surface;
a microscope unit for measuring the topography of deformations of the recording surface to extract numerical properties of sections of the recording surface;
a classification unit for using the numerical parameters and the recording standard to classify the sections of the recording surface; and
a signal processing unit for extracting the digital data from the classification of the sections.